Processes and devices which merge silica optical devices with advanced silicon electronics in integrated form are desirable. This will enable, for example, "super chips" to be provided which interface between optical and electronic technologies.
To achieve this integration it is desirable to be able to produce devices which combine thin film waveguides, and/or other optical components, usually silica based, married to a semiconductor substrate containing an electronic device or devices, usually silicon based.
One problem to be overcome in the fabrication of such devices is development of low temperature deposition processes which would allow deposition of thin film waveguides directly on top of electronic devices in a semiconductor substrate, without damaging the electronic devices. Plasma enhanced chemical vapour deposition (PECVD) is an attractive option for such low temperature deposition processes. Over the years of commercial use in conventional VLSI fabrication it has proven to be reliable, clean and well matched to modern automated IC production.
Some work has been done on the PECVD fabrication of waveguides.
1. [K. Imoto and A. Hori: "High refractive index difference and low loss optical waveguide fabricated by low temperature processes", Electron. Lett., Vol. 29, 1993, pp. 1123-1124.] PA0 2. [Franco Bruno, Massimo del Guidice, Roberto Recca, and Francesco Testa: "Plasma enhanced chemical vapour deposition of low-loss SiON optical waveguides at 1.5-.mu.m wavelength", Appl. Opt., Vol. 30, 1991, pp. 4560-4564.] PA0 3. [E. S. Bulat, M. Tabasky, B. Tweed, C. Herrick, S. Hapkin, N. J. Lewis, D. Obias, and T. Fitzgerald: "Fabrication of waveguides using low-temperature plasma processing techniques", J. Vac. Sci. Technol., Vol. A11, 1993, pp. 1268-1274.] PA0 4. [Q. Lai, J. S. Gu, M. K. Smit, J. Schmid and H. Melchior: "Simple technologies for fabrication of low-loss silica waveguides:, Electron. Lett. Vol., 28, 1992, pp. 1000-1001.] PA0 5. [G. Grand, J. P. Jadot, H. Danis, S. Valette, A. Fournier, A. M. Crouillet: "Low-loss PECVD silica channel waveguides for optical communications", Electron. Lett., Vol. 26, 1990, pp. 2135-2137.] PA0 6. [K. Kapser, C. Wagner, and P. P. Deimel: "Rapid deposition of high-quality silicon-oxynitride waveguides", IEEE Trans. Phot. Techn. Lett., Vol 3, 1991, pp. 1096-1098.] PA0 7. [C. M. Horwitz, S. Boronkay, M. Gross and K. E. Davies: "Hollow cathode etching and deposition", J. Vac. Sci. Technol, Vol. A6, 1988, pp. 1837-1844.]
These prior art processes have utilised nitrogen as a refractive index increasing dopant and/or nitrous oxide as an oxidant for silane. Such waveguides suffer, however, from high (3 to 10 dB/cm) losses in the wavelength range 1.50 to 1.55 .mu.m due to a large absorption peak in this region. These losses can only be reduced by annealing the fabricated device at temperatures around 1000.degree. C., which would destroy any electronic circuits in an associated semiconductor substrate. These processes do not, therefore, allow production of practical integrated opto-electronic devices. Nitrous oxides has always been used in prior art processes as an oxidant for silane. This is because it gives better thickness uniformity and is able to be used under high pressures, so as to optimise productivity.